Four valves in the heart serve to direct the flow of blood through the two sides of the heart in a forward direction. The mitral valve, located between the left atrium and the left ventricle, and the aortic valve, located between the left ventricle and the aorta, constitute the systemic portion of the heart. These two valves direct oxygenated blood coming from the lungs through the left side of the heart into the aorta for distribution throughout the body. The right side of the heart includes the tricuspid valve, located between the right atrium and the right ventricle, and the pulmonary valve, located between the right ventricle and the pulmonary artery. These two valves direct de-oxygenated blood returning from the body through the right side of the heart into the pulmonary artery for distribution to the lungs, where it again becomes re-oxygenated to begin its circuit anew.
Heart valves are passive structures having leaflets that simply open and close in response to differential pressures on either side of the particular valve. The mitral valve has two leaflets and the tricuspid valve has three. The aortic and pulmonary valves are sometimes referred to as semilunar valves because of the appearance of their three leaflets; these leaflets are shaped somewhat like a half-moon and are sometimes termed cusps.
The leaflets and surrounding elements of each valve vary with the function of the heart it supports. The atrioventricular valves, otherwise known as mitral (in the left chamber of the heart) and tricuspid (in the right chamber of the heart), are generally a continuum extending from the myocardium or muscular wall of the lower chambers, through the papillary muscles, to which is attached a confluence of tendinous rope-like elements, known as chordae tendinae, that are attached to the edges and undersurface of the differently shaped leaflets which open to allow flow and close to stop flow. The leaflets terminate at a ring-like structure usually known as an annulus, which is part of the fibrous skeleton of the heart.
When the left ventricular wall relaxes, the ventricular chamber enlarges and draws in blood from the atrium as the leaflets of the mitral valve separate, opening the valve. Oxygenated blood flows in a downward direction through the valve, to fill the expanding ventricular cavity. Once the left ventricular cavity has filled, the left ventricle contracts, causing a rapid rise in the left ventricular cavity pressure. This causes the mitral valve to close and opens the aortic valve, allowing oxygenated blood to be ejected from the left ventricle into the aorta. The chordae tendineae of the mitral valve prevent the mitral leaflets from prolapsing back into the left atrium when the left ventricular chamber contracts. The three leaflets, chordae tendineae, and papillary muscles of the tricuspid valve function in a similar manner, in response to the filling of the right ventricle and its subsequent contraction.
The cusps of the aortic valve respond passively to pressure differentials between the left ventricle and the aorta. When the left ventricle contracts, the aortic valve cusps open to allow the flow of oxygenated blood from the left ventricle into the aorta. When the left ventricle relaxes, the aortic valve cusps reassociate to prevent blood, which has entered the aorta from leaking (regurgitating) back into the left ventricle. The pulmonary valve cusps respond passively in the same manner in response to relaxation and contraction of the right ventricle in moving de-oxygenated blood into the pulmonary artery and thence to the lungs for re-oxygenation. These semilunar valves do not require associated chordae tendineae or papillary muscles.
Stenosis is one problem that heart valves may develop in which a valve does not open properly, another is insufficiency, or regurgitation, where a valve fails to close properly. In addition, a bacterial or fungal infection may require that a heart valve be surgically repaired or replaced. Sometimes such a problem can be treated by surgical repair of a valve; however, often a valve is too diseased to repair and must be replaced. If a heart valve must be replaced, there are currently several options available, and the choice of a particular type of artificial valve depends on factors including the location of the valve, the age and other specifics of the patient, and the particular surgeon's experiences and preferences.
Replacement heart valves or heart valve prostheses have been produced for more than four decades. Such valves have been made from a variety of materials of biologic and artificial nature; as a result two distinct categories of the prostheses have evolved: biological and mechanical prosthetic heart valves. Mechanical or artificial valves are typically constructed from nonbiological materials, such as plastics, metals and other artificial materials which, while durable, are prone to blood clotting which increases the risk of an embolism. Anticoagulants which may be taken to prevent blood clotting can possibly complicate a patient's health due to increased risk of hemorrhage.
Biological or tissue valves are constructed from animal tissue, such as bovine, equine or porcine tissue, although some efforts have been made at using tissue from a patient for which the valve will be constructed. Tissue valves are often constructed by sewing leaflets of pig aortic valves to a stent to hold the leaflets in proper position, or by constructing valve leaflets from the pericardial sac of cows, horses or pigs and sewing them to a stent. The pericardium is a membrane that surrounds the heart and isolates it from the rest of the chest wall structures. Such porcine, equine or bovine tissue is chemically treated to alleviate antigenicity and to make them more durable. Additional treatments may be applied to avoid structural valve deterioration in the long-term due to calcification. One main advantage of tissue valves is that they do not cause blood clots to form as readily as do the mechanical valves; therefore, they do not absolutely require life-long systemic anticoagulation. The major disadvantage of tissue valves is that they lack the long-term durability of mechanical valves.
Aortic stenosis is a very common disease of the aortic valve in the left ventricle of the heart in people above seventy years old and occurs more and more frequently as the subject gets older. When the aortic valvular orifice becomes tightly stenosed, blood can no longer be freely ejected from the left ventricle. As a result, the left ventricle has to markedly increase its ventricular chamber pressure to discharge blood past the stenosed aortic orifice; such causes a patient to have syncope, chest pain and difficulty in breathing.
Various surgical techniques that have been used to repair a regurgitant or damaged mitral valve include annuloplasty, quadrangular resection (narrowing the valve leaflets), and commissurotomy (cutting the valve commissures to separate the valve leaflets). The most common treatment for mitral stenosis and diseased aortic valve has been the replacement of an affected valve by a prosthetic valve via open-heart surgery by excising the valve leaflets of the natural valve and securing a replacement valve in the valve position, usually by suturing the replacement valve to the natural valve annulus. In instances where a patient is deemed operable only at too high a surgical risk, one alternative in valve stenosis has been to dilate the native valve with a balloon catheter to enlarge the valve orifice; however, such practice has experienced a high restenosis rate.
Generally, it would be desirable if heart valves could be replaced using minimally invasive techniques. Proposals have been made to remove a defective heart valve via an endovascular procedure, that is, a procedure where the invasion into the body is through a blood vessel, such as the femoral artery, and is carried out percutaneously and transluminally using the vascular system to convey appropriate devices to the particular body position to carry out the desired procedure. Angioplasty is also an example of such a procedure wherein a catheter carrying a small balloon at its distal end is manipulated through the body's vessels to a point where there is a blockage in a vessel. The balloon is expanded to create an opening in the blockage, and then deflated; the catheter and balloon are then removed. Such endovascular procedures have substantial benefits both from the standpoint of health and safety as well as cost. Such procedures require minimal invasion of the human body, and there is consequently considerable reduction and in some instances even elimination, of the use of a general anesthesia and much shorter hospital stays.
U.S. Pat. No. 6,168,614 to Andersen et al., the disclosure of which is incorporated herein by reference, discloses a heart valve prosthesis that can be implanted in the body by use of a catheter. The valve prosthesis includes a support structure or stent with a tissue valve connected to it that is delivered in a collapsed shape through a blood vessel. It is secured in expanded condition at a desired location in a blood vessel, e.g. downstream for the aortic valve.
Until recently, simple implantation of a valve prosthesis for the treatment of aortic stenosis without excising the native valve has been considered generally unrealistic because it was deemed difficult to simply superpose an implanted valve on the distorted stenosed native valve. More recently, however, procedures have been proposed, particularly for replacement of an aortic valve, such as those illustrated in U.S. Patent Application Publication No. 2005/0143809 to Salahieh et al., the disclosure of which is incorporated herein by reference, for endovascularly delivering an aortic prosthesis through the patient's aorta and implanting it within the confines of native valve. The prosthesis is delivered in a sheath to a location near the patient's native aortic valve and then expanded from its collapsed configuration to a deployed configuration. A variety of arrangements are described for deploying prostheses of various shapes and designs so that the prosthesis becomes implanted interiorly of the three native leaflets of the aortic valve, which are compressed radially outwardly.
Systems of this general type have shown promise and are considered to be attractive and accordingly, efforts are continuing to produce improvements in such prosthetic valves that can be minimally invasively implanted.
To repair or replace the mitral valve, a conventional procedure for approaching the left atrium has been employed by the use of intravascular catheterization from a femoral vein through the cardiac septal, which separates the right atrium and the left atrium. In some aspects, this intravascular procedure is not only dangerous and tedious because of long tortuous route, but it has limited use because of the catheter size suitable for intravascular insertion.
Overall, the use of a minimally invasive approach has a great number of advantages; an endovascular approach has generally been used. However, there is only limited space available within the vasculature; thus, the surgical field is often only as large as the diameter of a blood vessel. Consequently, the introduction of tools and prosthetic devices becomes greatly complicated, and the device to be implanted must be dimensioned and configured to permit it to be introduced into the vasculature, maneuvered therethrough, and then positioned at a desired location. In the majority of aged patients suffering from aortic stenosis, the aortic vessel and aortic arch are affected by calcified atheromatous plaques. Delivery of bulky tools and prosthetic devices retrograde through an atheromatous aortic vessel has increased risk of injuring of the atheromatous aortic wall with subsequent potential embolism and even aortic wall rupture.
Sterman et al., U.S. Pat. No. 6,283,127, the disclosure of which is incorporated herein by reference, discloses a minimally invasive system for facilitating intervention within the heart or great vessels without the need for a median sternotomy or other form of gross thoracotomy in order to try to substantially reduce trauma, risk of complications, recovery time, and pain for the patient. The surgical procedure is not endovascular, but is performed through percutaneous penetrations within intercostal spaces of the patient's rib cage, without cutting, removing, or significantly displacing any of the patient's ribs or sternum.
U.S. Pat. No. 6,869,444, the disclosure of which is also incorporated herein by reference, discloses the implantation of a prosthesis, which may include only a single leaflet for a mitral valve, through a barrel of an implantation device that is inserted either through the left atrial wall or through the apex of the heart.
U.S. Patent Application Publication No. 2005/0240200 to Bergheim, the disclosure of which is also incorporated herein by reference, discloses methods and systems for delivering a valve prosthesis to a site within the heart generally at or near the apex of the heart.
Therefore, it would be desirable to provide a delivery system for delivering a prosthetic heart valve to a patient's heart configured to be releasably folded or crimped inside a lumen of the delivery system through a percutaneous intercostal penetration of a patient's chest, or through an opening at a carotid artery, jugular vein, subclavian vein, femoral vein and other blood vessel.